The present invention relates in general to laser beam modulation, and is particularly directed to a new and improved mechanism for (Q-switch) modulating a high power laser, such as multimode laser having a randomly polarized output beam and large divergence angle. The Q-switch mechanism of the invention employs a plurality of longitudinal mode acousto-optic modulators, each of which has a relatively short, Raman Nath or near Raman Nath based non-coherent interaction length, and being optically cascaded such that the distance the randomly polarized laser beam travels through the modulators is effectively confined to the sum of the non-coherent interaction lengths thereof.
Acousto-optic Q-switches for controllably modulating (e.g., ON-OFF switching) randomly polarized laser beams have customarily been installed within the Fabry-Perot etalon cavity of the laser, and are controllably operative to variably and selectively modify the optical loss within the laser resonator cavity, thereby controlling initiation, amplitude and duration of the laser output beam. For xe2x80x98high-lossxe2x80x99 modulation, depletion in excess of 75% and usually in a range of at least 85-95% is necessary to prevent the laser cavity from lasing under high gain and intense pumping conditions.
For achieving high power, multimode operation is necessary. However, in a multimode operation, the laser beam is non-Gaussian and has a substantial divergence angle (on the order of five to ten milliradians or higher). This mandates that the interaction distance through the Q-switch medium be relatively long, in order to achieve high efficiency. This increase in modulator length is in conflict with the condition that, in order to satisfy the coherence requirement in an increased length acousto-optic medium, the acceptance angle must be decreased.
As a non-limiting example, for a continuous wave laser producing an output wavelength of 1.06 microns, a typical acousto-optic Q switch may be made of fused silica and operates at a frequency of 27 MHz. For longitudinal wave mode operation, the fused silica medium may have a height of 3.5 mm, a length of 42 mm, and an acousto-optic Q of 4.21. For shear wave mode operation, the fused silica device may have a height of 3.5 mm, a length of 42 mm, and an acousto-optic Q of 10.6.
Because the loss modulation efficiency for a longitudinal mode fused silica Q-switch is higher for vertical polarization than it is for horizontal polarization, it would seem that two orthogonally polarized devices might be employed to provide high loss modulation for a randomly polarized laser. However, because currently available Q switches have a relatively high acousto-optic Q value, they are not optimum, and are unable to effectively quench high power lasers whose outputs may be well over one hundred watts and have a divergence angle on the order of five milliradians or more.
Moreover, although both conventional longitudinal mode and shear mode Q-switches are capable of providing high loss modulation, as long as the laser beam is coincident with the Bragg angle of the switch, they suffer a significant reduction in efficiency.when the laser beam is not aligned with the Bragg angle. This results in a substantial reduction in loss modulation efficiency for high power, multimode laser applications, whose typical five milliradians or greater divergence angle causes Bragg angle misalignment of some portion of the beam.
While this problem might seem resolvable by placing two identical Q-switches in the laser cavity to take advantage of the cumulative loss modulation effect, its has been observed that installing two identical Q-switches in the resonant cavity of a high power, multi-mode laser results in a substantial reduction in output beam power. This laser output power reduction is due to the fact that the xe2x80x98windowsxe2x80x99 through the two Q-switches are of the same path length and parallel to each other along the beam axis. As a result, the etalon effect restricts the number of longitudinal modes that can be supported within the cavity, thereby causing a reduction in laser output power.
For an illustration of non-limiting examples of literature describing various conventional Q-switch arrangements, attention may be directed to the following U.S. Pat. Nos.: 5,268,911 to E. Young; 4,979,176 to E. Young et al; 5,197,074 to D. Emmons et al; Re. 34,192 to T. Baer; 4,930,901 to J. Johnson et al; 4,337,442 to M. Mauck; and 5,130,995 to W. Grossman et al.
In accordance with the present invention, the desire to provide a high modulation loss efficiency, high acceptance angle, Q-switch for a high power (multimode) laser, whose output beam is randomly polarized and has a substantial divergence angle is successfully achieved by a cascaded arrangement of orthogonally polarized longitudinal mode acousto-optic modulators, each having a relatively short, Raman Nath or near Raman Nath-based non-coherent interaction length. By relatively short, Raman Nath or near Raman Nath-based non-coherent interaction length is meant that the beam travel path through a respective modulator""s bulk material is considerably less than that required for proper Q-switch operation for a well collimated beam at a given power level, yet achieves a loss modulation efficiency that is sufficiently high to prevent the laser cavity from lasing under high gain and intense pumping conditions, even when the incident beam has a relatively large acceptance angle (on the order of five mrad or more).
The relatively short, Raman Nath or near Raman Nath Q-switch mechanism of the invention incorporates more Raman Nath or near Raman Nath interaction with the traveling acoustic wave in the bulk material. The laser beam is scattered into multiple orders, with the laser beam set at normal incidence, rather than being aligned with the Bragg angle, thereby allowing a relatively large acceptance angle (on the order of five mrad or more). This is very advantageous when the Q-switch is employed to extinguish. laser beams with a large angular divergence. Also, the relatively short Raman Nath or near Raman Nath interaction length means that the overall intra-cavity length can be reduced for narrower Q-switch pulse applications.
The cascaded AO Q-switch arrangement of the invention contains a plurality (e.g., pair) of orthogonally polarized longitudinal mode acousto-optic modulators, each of which has a prescribed, relatively short, Raman Nath or near Raman Nath interaction length, that physically and optically xe2x80x98stackedxe2x80x99 relatively close to one another, so that the overall beam travel distance therethrough is effectively confined to the sum of the respective non-coherent interaction lengths thereof. The modulators may be formed as discrete devices or they may be integrated within the same acousto-optic bulk material.
Due to the substantial power level of a high power laser with which the invention is employed, there is unavoidable heating of the bulk material, which usually results in a shift in the Bragg angle. Although this adversely affects the performance of a device having a long interaction length, it has minimal effect on the shorter interaction length of the Q-switch modulator of the invention. Moreover, the actual heating of the bulk material is countered by the incorporation of cooling fluid channels in a heat sink coupled in thermal communication with the bulk material.
The inherent heating of the bulk material that results from the substantial power level of the laser may be effectively countered by incorporating cooling fluid channels in a heat sink support block to which the bulk material of each of the cascaded orthogonally polarized Q-switch devices is mounted. The support and cooling arrangement for a cascaded arrangement may be configured to support and cool the modulators as discrete components, or as devices integrated within the same acousto-optic bulk material. In the integrated bulk support and cooling arrangement, in addition to the provision of an internal channel for the passage of a cooling fluid (e.g. water), the heat sink components of the packaging architecture for the two integrated wide acceptance angle Q-switch modulators are configured to be effectively mechanically floating with respect to the support housing, so as to reduce the build-up of thermally induced stresses.
In addition to being non-coherently driven in a longitudinal mode configuration, the AOM devices may be non-coherently driven with respectively different frequencies for shear mode acoustic waves. In this mode of operation, the minimum frequency difference requires a time bandwidth product of one or greater.